1. Field of the Invention
The present invention relates to an oxide magnetic material having soft magnetism, and more particularly, to a Mnxe2x80x94Zn ferrite suitable for use as a switching power transformer, a rotary transformer and the like, and to a production process thereof.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mnxe2x80x94Zn ferrite. This Mnxe2x80x94Zn ferrite of the prior art usually has a basic component composition containing 52 to 55 mol % of Fe2O3 on the average exceeding 50 mol % which is the stoichiometric composition, 10 to 24 mol % of ZnO and the remainder of MnO. And the Mnxe2x80x94Zn ferrite is usually produced by mixing the respective material powders of Fe2O3, ZnO and MnO in a prescribed ratio, subjecting the mixed powders to the respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a prescribed shape, and then sintering the resulting product at 1200 to 1400xc2x0 C. for 2 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is controlled to a low level by supplying nitrogen. The Mnxe2x80x94Zn ferrite is sintered in the reducing atmosphere in order to produce a part of Fe3+ thereby forming Fe2+. This Fe2+ has positive crystal magnetic anisotropy and cancels negative crystal magnetic anisotropy of Fe3 thereby enhancing soft magnetism.
Amount of the above-mentioned Fe2+ formed depends on relative partial pressures of oxygen in sintering and cooling after the sintering. Therefore, when the relative partial pressure of oxygen is improperly set, it becomes difficult to ensure excellent soft magnetic properties. Thus, in the prior art, the following expression (1) has been experimentally established and the relative partial pressure of oxygen in sintering and in cooling after the sintering has been controlled strictly in accordance with this expression (1):
xe2x80x83log Po2=xe2x88x9214540/(T+273)+bxe2x80x83xe2x80x83(1)
where T is temperature (xc2x0 C.), Po2 is a relative partial pressure of oxygen, and b is a constant. Usually, the constant b is set to 7 to 8. The fact the constant b is set to 7 to 8 means that the relative partial pressure of oxygen in the sintering must be controlled in a narrow range, which makes the sintering treatment very troublesome thereby increasing the production costs.
In recent years, with miniaturization and performance improvement of electronic equipments, there is an increasing tendency that frequencies of processing signals become higher. Thus, a magnetic material having excellent magnetic properties even in a higher frequency region has been needed.
However, when the Mnxe2x80x94Zn ferrite is used as a magnetic core material, as a frequency region applied becomes higher, an eddy current flows to result in a larger loss. Therefore, to extend the upper limit of the frequency at which the Mnxe2x80x94Zn ferrite can be used as a magnetic core material, an electrical resistivity of the material must be made as high as possible. However, since the above-mentioned usual Mnxe2x80x94Zn ferrite contains Fe2O3 in an amount larger than 50 mol % which is the stoichiometric composition, a large amount of Fe2+ ion is present thereby making easy the transfer of electrons between the above-mentioned Fe3+ and Fe2+ ions. Thus, the electrical resistivity of the Mnxe2x80x94Zn ferrite is in the order of about 1 xcexa9m (order of one digit) or less. Accordingly, an applicable frequency is limited to about several hundred kHz maximum, and in a frequency region exceeding this limit, permeability (initial permeability) is significantly lowered and the properties of the soft magnetic material are completely lost.
In order to increase an apparent resistance of the Mnxe2x80x94Zn ferrite, in some cases, CaO, SiO2 and the like are added as additive to impart a higher resistance to grain boundaries and at the same time the Mnxe2x80x94Zn ferrite is sintered at as low as about 1200xc2x0 C. to diminish grain sizes from their usual dimension, about 20 xcexcm, to 5 xcexcm, thereby taking measures to increase the ratio of the grain boundary. However, even if such measures are adopted, it is difficult to obtain an electrical resistivity exceeding 1 xcexa9m order because resistance of the grain boundary itself is low, and the above-mentioned measures fall short of a thorough solution.
Further, a Mnxe2x80x94Zn ferrite in which, for example, CaO, Sio2, SnO2 and TiO2 are added to obtain a higher resistance has been developed and is disclosed in Japanese Patent Application No. Hei 9-180925. However the electrical resistivity of the Mnxe2x80x94Zn ferrite is as low as 0.3 to 2.0 xcexa9m, which is insufficient for use in a high frequency region. Similarly, a Mnxe2x80x94Zn ferrite to which SnO2 and the like are added is disclosed in EPC 1,304,237. The Mnxe2x80x94Zn ferrite described in this EPC patent contains as much as 3 to 7 mol % of Fe2+. An electrical resistivity depends on amount of Fe2+ as described above, and the electrical resistivities of the Mnxe2x80x94Zn ferrite in this EPC patent cannot exceed the electrical resistivities of a usual Mnxe2x80x94Zn ferrite of the prior art.
On the other hand, Mnxe2x80x94Zn ferrites which exhibit a higher resistance by containing less than 50 mol % of Fe2O3, have been developed for use as a core material for a deflection yoke and are disclosed in Japanese Patent Application Laid-open Nos. Hei 7-230909, Hei 10-208926, Hei 11-199235 and the like.
However, judging from the fact that their usage is a core material for a deflection yoke and from the examples of the invention described in each publication, the Mnxe2x80x94Zn ferrites described in any of the publications are ferrite materials intended for applications in a frequency region of 64 to 100 kHz. The purpose of setting Fe2O3 content to less than 50 mol % for a high electrical resistivity is to enable a copper wire to be wound directly around a core for a deflection yoke. In the ferrite materials, excellent magnetic properties are not obtained in such a high frequency region as exceeding 1 MHz. Thus, it does not enable the ferrites to be used as a magnetic core material in such a high frequency region as exceeding 1 MHz to only set the Fe2O3 content to less than 50 mol % for a high electrical resistivity.
Further, a Mnxe2x80x94Zn ferrite containing 50 mol % or less of Fe2O3 to which 1.3 to 1.5 mol % of CoO is added in order to decrease the temperature coefficient of initial permeability is disclosed in Japanese Examined Patent Publication No. Sho 52-4.753. This Mnxe2x80x94Zn ferrite is not intended for obtaining a property of low loss in such a high frequency region as exceeding 1 MHz, either, and relative partial pressure of oxygen in sintering and cooling after the sintering is not strictly controlled.
The present invention has been made in consideration of the above-mentioned conventional problems. An object of the present invention is to provide a Mnxe2x80x94Zn ferrite that has, of course, excellent magnetic properties and also has both a higher electrical resistivity than 1 xcexa9m order (a single digit order) and a low core loss in such a high frequency region as exceeding 1 MHz, and a production process as well, by which such a Mnxe2x80x94Zn ferrite can be obtained easily and inexpensively.
One of the Mnxe2x80x94Zn ferrites according to the present invention for attaining the above-mentioned object is characterized in that its basic component composition includes 44.0 to 49.8 mol % of Fe2O3, 6.0 to 15.0 mol % of ZnO (15.0 mol % is excluded), 0.1 to 3.0 mol % of CoO, 0.02 to 1.20 mol % of Mn2O3 and remainder of MnO, and that the average grain size is less than 10 xcexcm.
Another Mnxe2x80x94Zn ferrite according to the present invention is characterized in that its basic component composition includes 44.0 to 49.8 mol % of Fe2O3, 6.0to 15.0 mol % of ZnO (15. 0 mol % is excluded), 0.1 to 3.0 mol % of CoO, 0.1 to 6.0 mol % of CuO, 0.02 to 1.20 mol % of Mn2O3 and remainder of MnO, and that the average grain size is less than 10 xcexcm.
Still another Mnxe2x80x94Zn ferrite according to the present invention may contain as additive, in addition to the basic component compositions of the above-described two inventions, at least one component of 0.005 to 0.200 mass % of CaO, 0.005 to 0.050 mass % of SiO2, 0.010 to 0.200 mass % of ZrO2, 0.010 to 0.200 mass % of Ta2O5, 0.010 to 0.200 mass % of HfO2 and 0.010 to 0.200 mass % of Nb2O5.
And, a production process according to the present invention to attain the above-mentioned object is characterized in that mixed powder whose components are adjusted so as to have the composition of the above-mentioned Mnxe2x80x94Zn ferrite is pressed, then sintered and cooled after the sintering down to 500xc2x0 C. or lower in an atmosphere of a relative partial pressure of oxygen obtained by using an optional value selected from a range of 6 to 12 as a constant b in the aforementioned expression (1).
In a usual Mnxe2x80x94Zn ferrite of the prior art, Fe2O3 content is larger than 50 mol % that is the stoichiometric composition, as described above. In order not to permit this excessive Fe2O3 to get precipitated as hematite, sintering and cooling must be conducted under a condition where a relative partial pressure of oxygen is reduced to a significantly lower level by flowing nitrogen, that is a condition where the constant b in the aforementioned expression (1) is set to 7 to 8. On the other hand, since in a Mnxe2x80x94Zn ferrite of the present invention, Fe2O3 content is 44.0 to 49.8 mol % that is less than 50 mol %, hematite hardly precipitates. Thus, even if a range of relative partial pressure of oxygen in sintering is somewhat widened, excellent magnetic properties can be obtained. Further, in the conventional Mnxe2x80x94Zn ferrite that contains more than 50 mol % of Fe2O3, about 3.0 mol % of Fe2+ exists. On the other hand, in the Mnxe2x80x94Zn ferrite of the present invention, the Fe2+ content is as low as 0.1 to 0.7 mol %. Accordingly, the electrical resistivity of the Mnxe2x80x94Zn ferrite of the present invention is very high. Therefore, even in a high frequency region, an eddy current is not increased so much and an excellent initial permeability can be obtained. However, if the Fe2O3 content is too small, the saturation magnetization is deteriorated. Thus, at least 44.0 mol % of Fe2O3 must be contained.
ZnO contained as main component affects the Curie temperature and the saturation magnetization. Too small amount of ZnO reduces the initial permeability, but on the contrary, too large amount of ZnO deteriorates the saturation magnetization and lowers the Curie temperature. Thus, since ferrite for power transformer is often used in an environment of about 80 to 100xc2x0 C., it is particularly important that it has a high Curie temperature and a high saturation magnetization. Accordingly, ZnO content in the ferrite is set to the above-mentioned range of 6.0 to 15.0 mol % (15.0 mol % is excluded).
Since Co2+ in CoO has a positive crystal magnetic anisotropy, CoO can cancel out a negative crystal magnetic anisotropy of Fe3+ even if Fe2+ having a positive crystal magnetic anisotropy exists only in a small amount. Further, Co2+ has an effect of reducing a loss in a high frequency region by generating an induction magnetic anisotropy. However, when CoO content is too small, the effect is small. On the contrary when CoO content is too large, the magnetostriction increases and the initial permeability is decreased. Thus, CoO content is set to a range of 0.1 to 3.0 mol %.
A manganese component in the above-mentioned ferrite exists as Mn2+ and Mn3+. Since Mn3+ strains a crystal lattice thereby significantly lowering the initial permeability, Mn2O3 content is set to 1.20 mol % or less. However, if Mn2O3 content is too small, the electrical resistivity is significantly decreased. Thus, at least 0.02 mol % of Mn2O3 must be contained in the ferrite.
In the present invention, the basic component composition may further include CuO. This CuO has an effect of enabling the ferrite to be successfully sintered at a low temperature. However, if the content thereof is too small, the effects are small. On the contrary if the content is too large, a core loss increases. Accordingly, the content is set to 0.1 to 6.0 mol %.
In the present invention, CaO, SiO2, ZrO2, Ta2O5, HfO2 or Nb2O5 can be contained as additive. These additives have an action of accelerating crystal grain growth and are effective in diminishing an average grain size to less than 10 xcexcm. However, if their contents are too small, the effect is small, and on the contrary if their contents are too large, abnormal grain growth occurs. Thus, CaO content is set to 0.005 to 0.200 mass %, SiO2 content is set to 0.005 to 0.050 mass %, ZrO2 content is set to 0.010 to 0.200 mass %, Ta2O5 content is set to 0.010 to 0.200 mass %, HfO2 content is set to 0.010 to 0.200 mass %, and Nb2O5 content is set to 0.010 to 0.200 mass %.
The core loss of ferrite in a high frequency region comprises mainly eddy-current loss and residual loss. As described above, the Mnxe2x80x94Zn ferrite according to the present invention has a very high electrical resistivity and a small eddy-current loss. Further, since the Mnxe2x80x94Zn ferrite has a small average grain size of less than 10 xcexcm, number of magnetic domain walls in a crystal grain is decreased, whereby the residual loss can be significantly decreased.
In the present invention, amount of Mn3+ is controlled by conducting the sintering and the cooling after the sintering in an atmosphere of the relative partial pressure of oxygen obtained by using an optional value in a range of 6 to 12 as the constant b in the expression (1) described above. When a value larger than 12 is selected as the constant b, the amount of Mn3+ in the ferrite increases to exceed 1.20 mol % whereby the initial permeability is rapidly decreased. Therefore, the amount of Mn3+ in the ferrite must be decreased in order to increase the initial permeability. Thus, it is desirable to select a small value as the constant b. However, when a value smaller than 6 is selected, the electrical resistivity is significantly decreased by the fact that amount of Fe2+ increases or amount of Mn3+ decreases to be too small. Accordingly, the constant b is set to at least 6.
In the production of a Mnxe2x80x94Zn ferrite, respective raw material powders of Fe2O3, ZnO, CoO, Mn2O3 and MnO as main component are previously weighed for a prescribed ratio, and mixed. Then this mixed powder is calcined and finely milled. The calcining temperature slightly differs depending on target composition and an appropriate temperature can be selected from a range of 800 to 1000xc2x0 C. A general purpose ball mill can be used for fine milling of the calcined powder. When CaO, SiO2, ZrO2, Ta2O5, HfO2 or Nb2O5 is made to be contained as additive, proper amount of powders of the respective additives are added to the calcined and fine milled powder and mixed to obtain a mixture having a target composition, which is then granulated and pressed in accordance with a usual ferrite production process, and sintered at 1000 to 1400xc2x0 C. A process of adding a binder such as polyvinyl alcohol, polyacrylamide, methyl cellulose, polyethylene oxide, glycerin or the like can be used for the granulation, and a process of applying a pressure of, for example, 80 MPa or more can be used for the pressing.
In the above-mentioned sintering and cooling after the sintering, a relative partial pressure of oxygen is controlled by flowing inert gas such as nitrogen gas or the like into a sintering furnace. In this case, an optional value in a range of 6 to 12 can be selected as the constant b in the aforementioned expression (1), which provides a larger allowance as compared to the constant b (7 to 8) selected in a case where a usual Mnxe2x80x94Zn ferrite of the prior art containing more than 50 mol % of Fe2O3 is sintered, so the relative partial pressure of oxygen can be easily controlled. Further, in this case, the cooling after the sintering needs to be performed in accordance with the above-mentioned expression only until the temperature gets down to 500xc2x0 C. because the reaction of oxidation or reduction, at a temperature lower than 500xc2x0 C., can be ignored independent of relative partial pressures of oxygen.